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Eversion 101 an Introduction to Inside-Out Objects

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Eversion 101 an Introduction to Inside-Out Objects Eversion 101 An Introduction to Inside-Out Objects David Golden [email protected] June 26, 2006 YAPC::NA Image source: Michal Kosmulski Copyright © 2006 David A. Golden An introduction to the inside-out technique Inside-out objects first presented by Dutch Perl hacker Abigail in 2002 – Spring 2002 – First mention at Amsterdam.pm, – June 28, 2002 – YAPC NA "Two alternative ways of doing OO" – July 1, 2002 – First mention on Perlmonks Gained recent attention (notoriety?) as a recommended best practice with the publication of Damian Conway's Perl Best Practices Offer some interesting advantages... but at the cost of substantial complexity – Big question: Do the benefits outweight the complexity? Agenda for this tutorial: – Teach the basics – Describe the complexity – Let you decide 1 Copyright © 2006 David A. Golden Eversion 101 Lesson Plan: Five C's 001 Concepts 010 Choices 011 Code 100 Complexity 101 CPAN 2 Image sources: Michal Kosmulski,Bill Odom Copyright © 2006 David A. Golden 001 Concepts Image source: Michal Kosmulski Copyright © 2006 David A. Golden Three ideas at the core of this tutorial 1. Encapsulation using lexical closure 2. Objects as indices versus objects as containers 3. Memory addresses as unique identifiers TIMTOWTDI: Everything else is combinations and variations 4 Copyright © 2006 David A. Golden 'Classic' Perl objects reference a data structure of properties Hash-based object $obj = bless {}, "Some::Class"; Object 1 name rank serial_no Array-based object $obj = bless [], "Some::Class"; Object 2 0 1 2 3 5 Image source: Michal Kosmulski Copyright © 2006 David A. Golden Complaint #1 for classic objects: No enforced encapsulation Frequent confusion describing the encapsulation problem – Not about hiding data, algorithms or implementation choices – It is about minimizing coupling with the code that uses the object The real question: Culture versus control? – Usually a matter of strong personal opinions – Advisory encapsulation: 'double yellow lines' – Enforced encapsulation: 'Jersey barriers' The underlying challenge: Tight coupling of superclasses and subclasses – Type of reference for data storage, e.g. hashes, array, scalars, etc. – Names of keys for hashes – 'Strong' encapsulation isn't even an option 6 Copyright © 2006 David A. Golden Complaint #2: Hash key typos (and proliferating accessors) 1 A typo in the name of a property creates a bug, not an error – Code runs fine but results aren't as expected $self->{naem} = 'James'; print $self->{name}; # What happened? Accessors to the rescue (?!) – Runtime error where the typo occurs – Every property access gains function call overhead $self->naem('James'); # Runtime error here print $self->name(); My view: accessor proliferation for typo safety is probably not best practice – Private need for typo safety shouldn't drive public interface design – Couples implementation and interface 1 Locked hashes are another solution as of Perl 5.8 7 Copyright © 2006 David A. Golden Eureka! We can enforce encapsulation with lexical closure Class properties always did this Some::Class package Some::Class; my $counter; sub inc_counter { my $counter my $self = shift; inc_counter $counter++; 1 } 2 Damian Conway's flyweight pattern my @objects my @objects; new 0 1 2 3 sub new { my $class = shift; my $id = scalar @objects; name $objects[$id] = {}; rank return bless \$id, $class; get_name } serial_no sub get_name { my $self = shift; return $objects[$$self]{name}; } 2 A brief version of this was introduced in Advanced Perl Programming, 1st edition as ObjectTemplate 8 Copyright © 2006 David A. Golden 'Inside-Out' objects use an index into lexicals for each property Some::Class my %name Lexical properties Object 1 give compile-time Object 2 typo checking new Object 3 under strict! Object 4 my %rank # Correct: Object 1 $name{ $$self }; get_name Object 2 # Compiler error: Object 3 $naem{ $$self }; Object 4 my %serial_no do_stuff Object 1 Object 2 Object 3 Object 4 9 Image source: Michal Kosmulski Copyright © 2006 David A. Golden Review: 'Classic' versus 'Inside-Out' Classic: Objects as containers – Object is a reference to a data structure of properties – No enforced encapsulation – Hash-key typo problem Object 1 name rank serial_no Inside-Out: Objects as indices – Object is an index into a lexical data structure for each property – Enforced encapsulation using lexical closure – Compile-time typo protection my %name Object 2 Object 1 Object 2 Object 2 Index Object 3 Object 4 10 Image source: Michal Kosmulski Copyright © 2006 David A. Golden 010 Choices Image source: Michal Kosmulski Copyright © 2006 David A. Golden What data structure to use for inside-out properties? Object 2 Object 2 Index ? 12 Copyright © 2006 David A. Golden What data structure to use for inside-out properties? my %name Object 1 Object 2 Object 2 Object 2 Object 3 Index Object 4 my @name How to decide? 0 1 2 3 Array – Fast access – Index limited to sequential integers – Needs DESTROY to recycle indices to prevent runaway growth of property arrays Hash – Slow(er) access – Any string as index – Uses much more memory (particularly if keys are long) – Needs DESTROY to free property memory to avoid leakage 13 Copyright © 2006 David A. Golden What index? (And stored how?) Sequential number, stored in a blessed scalar – Tight coupling – subclasses must also use a blessed scalar – Subclass must use an index provided by the superclass – Unless made read-only, objects can masquerade as other objects, whether references to them exist or not! $$self = $$self + 1 my @name 0 1 2 3 Object 1 1 A unique, hard-to-guess number, stored in a blessed scalar (e.g. with Data::UUID) – Again, tight coupling – subclasses must also use a blessed scalar my %rank Object 2 8c2d4691 8c2d4691 14 Copyright © 2006 David A. Golden An alternative: use the memory address as a unique identifier Unique and consistent for the life of the object – Except under threads (needs a CLONE method) my %serial_no Object 3 0x224e40 0x224e40 ? 3 Several ways to get the memory address; only refaddr()is safe $property{ refaddr $self } Otherwise, overloading of stringification or numification can give unexpected results $property{ "$self" } $property{ $self } # like "$self" $property{ 0+$self } 3 Available in Scalar::Util 15 Copyright © 2006 David A. Golden Using the memory address directly allows 'black-box' inheritance When used directly as refaddr $self, the type of blessed reference no longer matters – Subclasses don't need to know or care what the superclass is using as a data type – Downside is slight overhead of refaddr $self for each access 4 Black-box inheritance – using a superclass object as the reference to bless – a.k.a. 'foreign inheritance' or 'opaque inheritance' – An alternative to facade/delegator/adaptor patterns and some uses of tied variables – Superclass doesn't even have to be an inside-out object use base 'Super::Class'; sub new { my $class = shift; my $self = Super::Class->new( @_ ); bless $self, $class; return $self; } There is still a problem for multiple inheritance of different base object types 4 Thanks to Boston.pm for name brainstorming 16 Copyright © 2006 David A. Golden These choices give four types of inside-out objects 9 1. Array-based properties, with sequential ID's stored in a blessed scalar – Fast and uses less memory – Insecure unless index is made read-only – Requires index recycling – Subclasses must also use a blessed scalar – no black-box inheritance ? 2. Hash-based properties, with a unique, hard-to-guess number stored in a blessed scalar – Slow and uses more memory – Robust, even under threads – Subclasses must also use a blessed scalar – no black-box inheritance 8 3. Hash-based properties, with the memory address stored in a blessed scalar – Subclasses must also use a blessed scalar – no black-box inheritance – Combines the worst of (2) and (4) for a slight speed increase 9 4. Hash-based properties, with the memory address used directly – Slow and uses more memory – Black-box inheritance possible – Not thread-safe unless using a CLONE method 17 Copyright © 2006 David A. Golden 011 Code Image source: Michal Kosmulski Copyright © 2006 David A. Golden File::Marker: a simple inside-out objects with black-box inheritance Key Features Useable directly as a filehandle (IO::File) without tying $fm = File::Marker->new( $filename ); $line = <$fm>; Set named markers for the current location in an opened file $fm->set_marker( $mark_name ); Jump to the location indicated by a marker $fm->goto_marker( $mark_name ); Let users jump back to the last jump point with a special key-word $fm->goto_marker( "LAST" ); Clear markers when opening a file $fm->open( $another_file ); # clear all markers 19 Copyright © 2006 David A. Golden File::Marker constructor use base 'IO::File'; use Scalar::Util qw( refaddr ); Full version of File::Marker available on CPAN my %MARKS = (); Uses strict and warnings Argument validation Error handling sub new { Extensive test coverage my $class = shift; Thread safety my $self = IO::File->new(); bless $self, $class; $self->open( @_ ) if @_; return $self; } sub open { my $self = shift; $MARKS{ refaddr $self } = {}; $self->SUPER::open( @_ ); $MARKS{ refaddr $self }{ 'LAST' } = $self->getpos; return 1; } 20 Copyright © 2006 David A. Golden File::Marker destructor and methods sub DESTROY { my $self = shift; delete $MARKS{ refaddr $self }; } sub set_marker { my ($self, $mark) = @_; $MARKS{ refaddr $self }{ $mark } = $self->getpos; return 1; } sub goto_marker { my ($self, $mark) = @_; my $old_position = $self->getpos; # save for LAST $self->setpos( $MARKS{ refaddr $self }{ $mark } ); $MARKS{ refaddr $self }{ 'LAST' } = $old_position; return 1; } 21 Copyright © 2006 David A. Golden Seeing it in action file_marker_example.pl textfile.txt use strict; this is line one use warnings; this is line two use File::Marker; this is line three this is line four my $fm = File::Marker->new( "textfile.txt" ); print scalar <$fm>, "--\n"; Output $fm->set_marker("line2"); this is line one -- print <$fm>, "--\n"; this is line two this is line three $fm->goto_marker("line2"); this is line four -- print scalar <$fm>; this is line two 22 Copyright © 2006 David A.
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